More than 70 percent of electricity in the United States comes from plants that require some water for cooling. Most plants that generate steam to make electricity use water to re-condense the steam for reuse. While many plants return that water to the environment, others use less water but lose that water to evaporation in the cooling process. In total, it’s a huge demand on fresh water supplies: More than 40 percent of fresh water used in the United States is withdrawn to cool power plants.

Originally plants were once-through systems, where the water was only used once and then put back into the ecosystem.

But other cooling techniques have been adopted in newer plants, though perhaps not fast enough given ongoing drought conditions. Because of environmental standards, most new plants use recirculating water systems and, to a lesser extent, dry cooling. The trend towards recirculating plants began in the 1960s, but the long lead-time on the switch over still means that many power plants still in use are once-through water systems. In the United States, 43 percent of plants do not reuse water, according to the EIA.